Resident Evil features several viruses and parasites throughout the games, but some infections have worse effects than others. However, the physical effects of the viruses aren't the only aspect to consider when ranking the worst infection. This is because some Resident Evil viruses can be treated if the infection is caught early enough, but that doesn't mean the treatment is easy or painless.
Resident Evil began with just the T-Virus, but the list of infections included in the series quickly grew. From the classic viruses at the beginning of the series to the more modern ones like the Mold seen in recent games, the universe of Resident Evil is filled with biohazards. While none of the viruses or parasites would be pleasant to be infected by, some are far worse in their effects than others.
10 Las Plagas Parasite
The majority of people infected with Las Plagas, such as the villagers in RE4 and RE5, don't suffer from severe mutations. However, the form of Las Plagas that the villagers are infected with allows somebody with the dominant variant to control them, as is the case with Saddler. But because of Luis' work, there's a way to remove the parasite if the infection is caught early enough, and the possibility of a treatment alone puts Las Plagas lower on the list than other infections, even if the treatment is painful.
9 T-Phobos Virus
A variant of the T-Virus, T-Phobos appears in Resident Evil Revelations 2. As its name might suggest, this virus is triggered by high levels of fear in the infected. While T-Phobos can have horrifying side effects, it might give those injected the highest chance of survival due to being triggered by extreme fear.
When the virus is triggered, it seems to affect victims differently depending on gender. Men undergo mutations, and women suffer from intense pain until they die. Even though T-Phobos being triggered almost guarantees the infected will die or mutate, for an average person, it's unlikely they'll encounter the levels of fear long enough for this to happen. Claire and Moira are infected with the virus in RE Rev 2, but even in the horrifying and stressful events that follow, the virus doesn't kill them.
8 Mold
Resident Evil 7's Eveline is at the center of the Mold, a fungal infection that lets her cause hallucinations in the infected. The effects of being infected with the Mold can vary, and some victims don't undergo visible physical mutations, which is seen in characters like Zoe, Mia, and Ethan. However, in other cases, the infected is consumed and killed by the Mold or undergoes physical mutations like Marguerite Baker. Even though it's possible to synthesize a vaccine for the Mold, there's an element of chance in how the infection will present itself that makes it more difficult to predict than the other RE viruses.
7 T-Veronica
T-Veronica is another strain of the Progenitor Virus that was developed by Alexia Ashford and appears in Resident Evil: Code Veronica. It's possible for the infected to maintain control over their mind, but only when certain conditions are met. It's also possible to avoid severe mutations without being cryogenically frozen for 15 years like Alexia by performing organ transplants on infected organs until the body assimilates with the virus, which is seen in Manuela's case. As a result, T-Veronica is a virus that's difficult to survive being infected with unless the victim has access to resources that most people wouldn't have.
6 Progenitor
At this point in the ranking, the best outcome is a swift death. The Progenitor Virus is incompatible with the majority of the population, which results in it having a high mortality rate for the infected. It's also the origin for many strains of viruses in the Resident Evil universe. If the infected is part of the small portion of the population that's compatible with the virus, then the result is superhuman abilities and intelligence. The reason that it isn't higher on the list with such a high mortality rate is that it seems like the infected's death isn't prolonged, and if the cells are revived after death, there isn't awareness from the victim.
5 T-Virus
The first Resident Evil introduced the iconic T-Virus. The T-Virus can infect both the living and the dead, reanimating the latter, but since the virus causes necrosis, the living who are infected become the same as the dead victims in a matter of time, resulting in them becoming what is seen as a traditional zombie. The virus causes brain damage and gangrene, but there's a delay before those effects set in, which is how characters like Jill Valentine were able to receive a vaccine and survive after being infected.
4 T-Abyss
T-Abyss is the strain of the T-virus seen in Resident Evil Revelations. Unlike the regular T-Virus, this version doesn't result in the infected turning into Zombies. Instead, they become Oozes, which are bloated creatures with suckers. Similar to how the T-Virus can cause the infected to become cannibals, Oozes lose body water and will seek out ways to replenish their water by taking it from other organisms. Despite developing bioweapons, Tricell contained T-Abyss as the company believed it was too dangerous to use as a bioweapon.
3 G-Virus
The G-Virus appears in Resident Evil 2, and it's the reason that the boss, William Birkin, mutates into various states throughout the game, becoming less human in appearance with each transformation. Unlike the T-Virus' ability to cause necrosis, the G-Virus heals infected cells and can revive dead cells, a long-lasting effect that's later seen in Sherry in the notoriously hated Resident Evil 6, even after she'd been treated for the virus. Between the extreme mutations and how easily spread it is, the G-Virus is highly dangerous.
2 C-Virus
The C-Virus is a combination of the G-Virus and T-Veronica that's seen in Resident Evil 6. The mutations caused by this virus are severe, and when the infected take damage, the virus' response is to mutate the damaged area further, although too much damage results in the infected incinerating as the virus tries to repair its body. The infected also retain a level of awareness, and while there's a vaccine for this virus, the vaccines usually don't reverse mutations that have already occurred.
1 Uroboros
Like the Progenitor Virus it's a strain of, Uroboros has a high mortality rate. Additionally, unlike some other infections, it doesn't have a vaccine, not that one would be useful as a treatment anyway given how quickly Uroboros mutates the infected. Uroboros is seen in Resident Evil 5, and it kills those who aren't compatible by causing uncontrolled mutations that essentially tear the infected's body apart. Since the infected seems to be aware while they're dying in this horrible manner, Uroboros earns its spot as the worst virus to be infected with from the Resident Evil universe.
Viruses are a focal point of Resident Evil. While the series started with the T-Virus, the infections seen in games now include parasites and fungal infections as well. As the effects of these infections range from mild and bearable to horrifying, it's difficult to predict what sort of infection will appear in the next Resident Evil.
